Process of heating caprolactam graft copolymers to increase melt strength

ABSTRACT

WHEN OLEFIN-MALEIC ANHYDRIDE COPOLYMERS ARE EMPLOYED AT LOW LEVELS IN HYDROLYTIC POLYMERIZATION OF CAPROLACTAM TO YIELD GRAFTED CAPROLACTAM COPOLYMERS, THE RESULTING POLYMERS UPON HEATING IN THE SOLID STATE AT A TEMPERATURE OF AT LEAST 140*C. INCREASE IN MELT STRENGTH TO VALUES CONSIDERABLY ABOVE THE MELT STRENGTH OF CONVENTIONAL POLYCAPROLACTAM OF EQUIVALENT MELT INDEX. THE RESULTING PRODUCTS HAVING HIGH MELT STRENGTH MAY BE ADVANTAGEOUSLY USED FOR THE MANUFACTURE OF EXTRUDED AND BLOV XXX OLDED ARTICLES.

(Grams Z frf/m/n.)

Feb.22,1972

R. P. ANDERSON ET AL v 3,644,571

PROCESS OF HEATING CAPROLACTAM GRAFT COPOLYMERS Examp/e 4 MEL T STRENG TH 4 5 8 l0 l2 MELT /NDEX (fimma/m mz'n.

INVENI0R5 ATTOENEK United States Patent 3,644,571 PROCESS OF HEATING CAPROLACTAM GRAFT COPOLYMERS TO INCREASE MELT STRENGTH Raymond P. Anderson and Francis R. Galiano, Oakland Park, Kans., assignors to Gulf Research & Development Company, Pittsburgh, Pa.

Filed Mar. 19, 1970, Ser. No. 20,905 Int. Cl. (308g 41/04 US. Cl. 260-857 G 6 Claims ABSTRACT OF THE DISCLOSURE DESCRIPTION OF THE INVENTION Long chain branching has been introduced into caprolactam polymers in a controlled manner by hydrolytic polymerization in the presence of certain copolymers. Conventional caprolactam polymers may be produced by hydrolytic polymerization using monomeric carboxylic acids or certain of their derivatives to promote the polymerization. For example, caprolactam may be polymerized in the presence of about 0.1 percent benzoic acid and about 1% water to yield a polymer of the conventional nylon 6 type. When the benezoic acid is replaced, for example, by from about 0.1 percent to 4 percent of a copolymer of ethylene, styrene or a l-olefin having from 3 to 8 carbon atoms with maleic anhydride, maleic acid, maleamic acid, or a mono ester of maleic acid, graft copolymers of caprolactam may be obtained which upon heating in the solid state for at least hours at temperatures above 140 C. acquire greatly improved melt strength.

Melt strength refers to the cohesive strength of the molten polymer. Normally caprolactam homopolymer may be made in high molecular weight with a great deal of resistance to flow so that the melt index is so low as to make extrusion almost unfeasible or the polymer can be made with lower molecular weight and reasonably high melt index but with a tendency to run like water when melted, so that extrusion and melt spinning are difficult. For purposes of injection molding and melt spinning, polymer with properties midway between these two extremes is usually employed, such products possessing a reasonably high melt index and sufiicient melt strength so that melt spinning and drawing of fibers becomes feasible. This may be done for extrusion also, but the comprise is generally not satisfactory, in that extrusion is necessarily slow and uneconomical and there are practical limitations of minimum film gauge.

There have been previously disclosed graft copolymers of caprolactam made by hydrolytic polymerization of caprolactam with copolymers or the hydrolyzed copolymers of maleic anhydride with olefinic monomers. We have discovered that polymers of this type, after washing and drying in the customary manner to prepare them for commercial use can be heated under vacuum in the solid state for a period of several hours with the result that melt strength is greatly improved with relatively small decrease in melt index, so that polymers which can easily be extruded are obtained.

Melt strength is the outstanding property which characterizes the thermoplastic materials customarily employed in blow molding of articles such as plastic bottles and containers. Polymers with high melt strength can be extruded, blown or subjected to mechanical working of different forms while molten and in a deformable plastic state but still having sufficient strength to resist tensile stresses. Melt strength may be measured in various ways. One method involves the measurement of changes in the diameter of a strand of extrudate from a melt index apparatus. See, for example, Combs et al. J. Appl. Poly. Sci. II, 747 (1967). The above method gives some indication of gross changes in the melt strength but does not serve to indicate adequately small differences, for the purpose of making quantitative comparisons. A method which measures the strength of molten nylon in tension has been found to be much more satisfactory. The instrument used for measuring melt strength in this way is described by Kaltenbacker et a l. in their article The Use of Melt Strength in Predicting the Processability of Polyethylene Coating Resins, TAPPI, vol. 50, p. 20, 1967. The equipment consists essentially of a melt index apparatus combined with an Instron apparatus or other screw driven device to drive the melt index piston at a specified rate, a means of varying the rate of drive and a strain gage measuring system to measure the tensional force in grams required to draw down the polymer extrudate at a feasible rate. For a conventional nylon resin the melt strength is dependent upon the melt index. A number of nylon resins were examined to determine the relationship between melt strength and melt index and the zone which includes these values is the shaded area in the drawing (FIG. 1). Also shown in FIG. 1 are the results obtained by heating various caprolactam graft copolymers in the solid state, lines being drawn from points indicating starting values of melt index and melt strength to other points which indicate the final values of these properties after heating for the number of hours indicated on the drawing. It can be seen that in each instance there was what might be regarded as an expected decrease of melt index but the increase in melt strength was exceptionally high when compared with conventional resins. Conventional nylon resins when heated under vacuum in the solid state yield products having melt index and strength values remaining within the normal zone shown in FIG. 1. See Example 5, for instance, which is indicated in FIG. 1.

The manner of carrying out the invention is discussed below:

Polymerization of caprolactam in presence of copolymer The molecular weight of the olefin-maleic anhydride copolymer may be varied from about 1000 to over 75,000. In actual practice, molecular weights from about 1000 to about 12,000 are preferred. Higher molecular weight olefin-maleic anhydride copolymers lead to high viscosity caprolactam graft copolymers. The high 'viscosities reached in such cases lead to practical problems in the manufacture of caprolactam graft copolymers as the high viscosity material is difficult to handle in conventional polymerization equipment. From a practical standpoint, it is more convenient to use a lower molecular weight olefin-maleic anhydride copolymer so that a product of normal viscosity and melt strength is obtained by the regular polymerization procedure. This material may then be subjected to further condensation by treating in the solid state to obtain a high melt strength polymer.

Post-polymerization of solid caprolactam graft copolymer The process of heating polycaprolactam in the solid state in order to increase the molecular weight and hence the viscosity is known, At temperatures above about 140 water and 1.0% of a styrene maleic anhydride copolymer, C. condensation reactions take place with the release of SMA 3000 A. Conditions used (reactor temperatures, levwater. In order to substantially increase the molecular els, residence times, etc.) are substantially equivalent to weight of the polymer, it is necessary to remove water. those used in making a conventional nylon resin catalyzed This is generally done by carrying out the heating under 5 by benzoic acid. The washed and dried product in a specific high vacuum such as -1 mm. of mercury absolute pressure. instance had a solution viscosity of 3.02, a melt index Alternative methods of removing the water such as pass- (235 C., 2160 g. load) of 11.4 grams/l0 minutes and a ing an inert gas over the solid polycaprolactam pellets melt strength of 0.7 grams. One portion of this material may be used. Another method of effecting the increase was heated for 12 hours at 165 C. and 1 mm. of mercury in molecular weight is by heating the solid polycapro- 1o vacuum to give a product having a relative solution vislactam in an inert liquid or its vapor so that the polymer cosity of 3.56, a melt index of 5.7 g./ minutes and a melt is heated by the liquid or its vapor and water is removed Strength of grams- A Second Portion Was heated for from the polymer by azeotropic distillation. 24 hours under the same conditions to give a product hav- Procedures for preparation of specific caprolactam ing a relative solution viscosity of 3.81, a melt index of graft copolymers and conversion to high tren th prod- 15 3.95 grams/ 10 minutes and a melt strength of 5.3 grams. ucts are illustrated below: The products of the method described above were melted and extruded into films and fibers, some of which Batch Ramon were then subjected to drawing to reduce thickness of the Four hundred grams of caprolactam, 8 g. of styreneextruded articles. The high melt strength resins possessed maleic anhydride copolymer (SMA 3000A) and 8 g. of readily apaprent advantages in the manufacture of blown water were placed in a one liter resin kettle equipped film. The bubble was readily formed and was of good stawith a condenser, stirrer and gas inlet and outlet. Argon bility. Faster extruder output rates could be employed than gas flow through the reactor was started one hour before with conventional nylon and higher film take-01f speeds heat was applied and was continued until after the polymwere possible. The film and fibers are somewhat stiffer erization was completed. The reaction mixture was heated than those m d f m a C t nal IeSiIl- The in to 200 C. and maintained at that temperature over a general, is less blocky and stiffer than that made from contwo hour period after which it was heated to 265 C. and ventional nylon, and has a unique crispness and transmaintained at this temperature until a total heating time l n ppear n e. Ex ru sh et of ppr xim ly 1- of 26 hours was reached. Cooling water was supplied to thickness is White and translucent in unpigmented the condenser during the first seven hours of heating after 30 form and y be readily awn Vacuum formed when which it was shut off and the stirrer removed. h d o softening temp ra re- After cooling, the raw olymer was r u d d The products of improved meltstrength made by the tracted with methanol and dried. The solution Viscosity, method of this invention are pertlclllarly useful melt index and melt strength of the extracted and dried truslen coating to w multl-leyered artwles on a vanety polymer were determined. of substrates, including other polymers, paper and metal Conversion by heating i th lid t t was i d foils and sheets. Included among these multilayered prodout in a drying pistol at 0.1-0.2 H using fl i ucts are nylon-coated aluminum foil and heat-scalable diisobutyl *ketone (B.P. 167-70 C.) as the constant temnylon-aluminum "P Y Y laminates 1156 a5 a perature h at source, heat scalable vapor barrier and heat-reflecting sheet in Properties of specific resins prepared by hydrolytic 40 Packaging and bulldmg CQBStTHCtIOPt nylon-Polyethylene bat h polymerization i h presence f various 1 lannante for meat packaging, abrasive coated cloth belts mers, along i h h ff t f post polymerization are w th extruded nylon coatings on the back s1des to reduce given in Table I. Included for the purpose of comparison fnctlon and Wear 111 {lylon'coated Pnnted P-' P and are d f r a Conventional resin card stock for book bindings and sheet metal with ex- TABLE I.EFFECT OF POST POLYMERIZATION OF CAPROLACTAM GRAFT COPOLYMERS Polymerization formulation Original resin properties 1 P t Post polymerized resin products OS Olefin copolymer Percent polymeri- EXBJIIDIB Percent capro- Solution Melt Melt zation Solution Melt Molt plumber Type Percent. Ht lactam viscosity index strength time, hr. viscosity index strength 2 SMA3420A 2 2 9s 2. 99 3 08 0.7 27 3. 84 2.8 6.4 2 2 05 2.82 12 1 ca. 1. 2 27 4.07 0.15 7.8 1 2 97 3. 06 10 2 0.8 24 3. 4.92 1.8 1 2 97 4. 04 6 so 1.1 12 5. 29 3. 09 10.0 0.07 1 99 2.84 9 1 0.0 24 3.51 4. 62 1.3 1 2 97 2.77 12 2 20 3. 9a 2. 54 4.4 1 2 97 2. 7s 14. 2 0. 5 12 3. 33 5. 16 2. 4 1 2 97 2. 70 14.2 0.5 24 3.54 3. 55 4.0 1 2 97 2.94 8.82 1.0 24 3.94 3. 34 5.0 1 2 97 2 77 12. 2 24 3.80 3.10 4. 9 2 2 so 2 94 8.8 1 0 24 3.94 3.34 5. 0

1 Measured on methanol extracted and vacuum dried (100 0., 0.2-1.0 mm. Hg, 18 hour) polymer.

2 Relative solution viscosity measured in 00% formic acid at a concentration of 1 g. of polymer per 100 ml. of solvent.

3 Grams per 10 minutes, measured at 235 0., 2,160 g. load.

4 Tensional force, in grams, required to draw down the molten extrudate from a melt index apparatus; plunger speed=0.5 inches per minute; draw down rate=20 feet per minute.

5 Post polymerizations were carried out in a drying pistol under refluxing diisobutyl ketone (B.P. 165-170 0.); Pressure=0.2-1 mm. Hg.

6 A conventional nylon resin.

No'rE.SMA3420A=Half ester of a styrene maleic anhydride copolymer, about wt. percent styrene; SMA3000A=Styrene-maleic anhydrde copolymer, about 75 wt percent. styrene; Polymac 6=Maleic anhydride l-hexene copolymer, about 46 wt. percent; l-hexene; EMA 11=Ethylene-maleic anhydride copolymer, about 22 wt. percent ethylene; EMA 22=Diacidform of an ethylene-malcic anhydride copolymer, about 19 wt. percent; ethylene Continuous reaction truded nylon coatings to reduce friction and wear against the die in cold forming operations. caprolactam graft copolymers may also be made ex- We claim:

perimentally on a small scale in a continuous pilot plant 1. The method of manufacturing modified caprolactam reactor. The continuous reactor consists of an enlarged graft copolymer of high melt strength comprising the step polymerization vessel on top of astem section. The polymof heating a caprolactam graft copolymer in the solid erization feed enters near the circumference of the prestate at a temperature of at least 140 C. but below the polymerization vessel, flows through a spiral of about ten melting point of the caprolactam graft copolymer for 11 turns t0 the camel 0f the Prepolymerization 165561 and period of at least 5 hours, sufiicient to effect an increase Passes downward through the Stem- The Polymerization in the melt strength of the graft copolymer, as measured feed consists, for example, of 97.7% caprolactam, 1.3% during melt extrusion, to a value above that of polycaprolactam having a substantially equivalent melt index, said caprolactam graft copolymer having been hydrolytically polymerized in the presence of from 0.1 percent to 4 percent of a copolymer having a molecular weight of from about 1,000 to about 75,000 of a monomer selected from maleic anhydride, maleamic acid, maleic acid, and mono esters of maleic acid with from about 20 up to about 80 weight percent of the total copolymer of a polymerizable monomer selected from the group consisting of styrene, ethylene and l-olefins having from 3 to 18 carbon atoms.

2. The method of claim 1 in which the polymerizable monomer is styrene.

3. The method of claim 1 in which said heating is done under vacuum.

4. The method of claim 2 in which the caprolactam graft copolymer is one made by hydrolytically polymerizing caprolactam in the presence of a polymer which has a molecular weight between 1000 and 12,000.

5. An extruded film made from the product of the method of claim 1.

6. An extruded fiber made from the product of the method of claim 1.

References Cited UNITED STATES PATENTS 3,539,664 11/1970 Kray 260-857 3,136,738 6/1964 Hedrick 260--857 3,243,476 3/ 1966 Black 260857 3,243,477 3/1966 Black 260-857 3,325,561 6/1967 Grillo 260857 3,3 88,186 6/1968 Kray 260-857 3,465,059 9/1969 Seven 260'857 PAUL LIEBERMAN, Primary Examiner US. Cl. X.R.

1l7155 R, 161 P; 161-190, 214, 216, 220, 227, 229; 260--78 S, 857 L, 857 U 

